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Wednesday, August 22, 2012

The meaning of the adaptation to tolerate lactose.

The question
that is most often asked about lactose tolerance is that if drinking milk is an
evolutionary adaptation shouldn't we continue to reap its fitness benefits? Although this question may be frowned upon by individuals who study evolutionary biology it is an important question to answer as it addresses the issues that are important to understanding of evolution; adaptation and natural selection.

I will
explore this question in two parts first I will explain the adaptation of
lactose tolerance and then I will explore how natural selection effects life history evolution.

Is Lactose tolerance an adaptation?

Lactose
tolerance occurs due to a genetic mutation(s) that confers an ability to digest
lactose from milk. Lactose tolerance appeared as a fortuitous mutation, yet it got
selected as a result of the fitness benefits it conferred; it had beneficial
effects on the adaptedness (absolute fitness) of individual humans within an
environmental context. Natural selection then occurred as a result of
differences in adaptedness of individuals within a specific population; i.e.
some individuals could drink milk into adulthood whereas others couldn’t. Over
time, those individuals that could drink milk into adulthood survived and
reproduced at relatively higher rate than those who could not. Over many
generations, through the process of
adaptation the trait for digesting lactose became adaptive within a spatio-temporal context.

Even though
the trait arose as a fortuitous mutation it became
adaptive as it created differences in absolute fitness and set stage for
natural selection to occur. Lactose tolerance gene was definitely an adaptation
to milk drinking in the spatio-temporal context it evolved in; must this mean
that this trait is as adaptive today as it was in our past? Does a trait’s
noble legacy justify its credentials in the present? This may not always be
true and the answer depends on the differences between past and present and the
nature of life history evolution.

It is
important at this point to understand that all adaptations have three aspects.

a)
Historical; an adaptive trait is present today in an organism because it
provided fitness benefits to its bearers and there was a selection for the adaptive trait.

b) Spatial:
An adaptive trait is a specific solution to a problem posed by a feature of a
given environment.

c)
Eventual: Adaptive traits have the ability to confer (present and future)
fitness benefits to its bearers.

Whether or
not an adaptive trait persists through time depends on a number of factors; the
environment being the most critical of them. If there are minimal changes in
the environment (over evolutionary time) such that the plasticity allows the
trait to persist, then the trait remains a legible solution to the problems
posed by the environment. On the contrary, if the environment has changed substantially enough so that the
plasticity of the trait does not buffer the changes, then the trait finds
itself in uncharted waters. Sober pointed out in “The Nature of Selection” that
“the benefits that account for a traits fixation need not persist into the
present. “The benefits that account for a traits fixation” are governed by the
environment (Sober, 1993).

If one
considers just the historical aspect of an adaptive trait then “a trait can be
an adaptation for performing some task, even though performing that task in the
present environment confers no benefit” (Sober, 1993).

This
statement can be misunderstood as the reader might comprehend that a traits noble
legacy justifies it present credentials. An adaptive trait remains an
adaptation no matter what happens. I would like to clarify what I think Sober
meant for the reader to read between the lines; an adaptive trait remains an
adaptive trait, retrospectively. A
trait is an adaptation just because it was
an adaptation in the spatial context
it evolved. The trait conferred fitness benefits to its eventual bearers.
The trait will still be called adaptive; the process which shaped the trait
will still be called adaptation, but retrospectively. Whether an adaptive
trait is adaptive today depends on the environmental context of the trait. If
the environment is substantially different from the environment in which the
trait evolved and became adaptive then the trait may not confer any fitness
benefits (Remember that the trait is still adaptive but…you guessed
it…retrospectively). If the environment changes, the trait remains adaptive
looking at the past; whether the trait still persists as an adaptation in the
future is another issue altogether.

There are
three scenarios a trait may experience if the environment changes; a) the trait
may not confer any fitness benefits, i.e. remain neutral and adaptive
retrospectively; b) the trait may happen to confer fitness benefits, persist
being adaptive; and c) the trait confers negative fitness benefits and become maladaptive.

Going back
to the adaptive trait of Lactose tolerance, we can either say that Lactose
tolerance was an adaptation in the
spatio-temporal context that favored its prevalence or lactose tolerance is an adaptation retrospectively. Both
statements are correct as long as you understand the definition of an
adaptation. The question therefore is not whether lactose tolerance is an
adaptation. It most definitely is! The question is whether lactose tolerance is
an adaptation in today’s environmental context?

In the arid
climates of Middle and Near east dairy has been suggested to be the major
source of clean uncontaminated water (Cook & Al-Torki, 1975). The individuals that could drink milk into
adulthood happened to have higher relative fitness than those who could not. In
northern Europe dairy was the source of calcium, this must have been crucial in
place where there is very little sunlight (Flatz & Rotthauwe, 1973).

Thus the
trait was selected and as the populations of individuals travelled across the
world the trait travelled with them. In certain places the trait might have
been still adaptive; yet, in other places the adaptive trait might have lost
its relevant environmental context. Rehydration through consumption of dairy is
not a necessity when you live next to a river, for example.

Thus we can
say that in certain populations in the past (and today) the adaptive trait of
lactose tolerance lost its relevance, as it could not confer fitness benefits.
It is lingering in our gene pool as a consequence of fitness benefits it
conferred to its bearers in their environmental
context. Those fitness benefits are not relevant in those populations whose
environmental context has changed. Thus lactose tolerance does not remain an
adaptation in those populations; however, that does not take away the adaptive
legacy of the trait. The trait has become a not-so-benign ghost of Christmas
past.

Lactose Tolerance is a mal-adaptation

Now that I
have established that a trait can lose its adaptive significance in a changed environmental context I will try
to make the case that this trait is not just a neutral adaptation but has the
potential to be transformed into a mal-adaptation in todays environmental
context. Lactose tolerance and the resultant consumption of milk have been
linked to many chronic diseases of the western civilization. Chronic
consumption of milk has been associated primarily with auto-immune diseases
(Type 1 Diabetes, Rheumatoid Arthritis, and Multiple Sclerosis) (Elliott, Harris, Hill, Bibby, & Wasmuth, 1999; Malosse, Perron,
Sasco, & Seigneurin, 1992), consumption of dairy has also been correlated
to prostate cancer and insulin resistance (Pereira et al., 2002; Qin et al., 2004). The ghost-adaptation of lactose tolerance is making
us realize the necessity of changing our ways (I couldn’t help myself there ).
Lactose tolerance today is a mal-adaptation
as the trait was previously selected for by
natural selection, lost its relevance, and now is becoming a crucial factor in
the development of chronic diseases.

Natural Selection and Life History Evolution

There is
another important aspect to the “adaptation” of consuming dairy into adulthood.
It is centered on the workings of natural selection.

Natural
selection operates most quickly on traits and behaviors that are expressed
during the reproductive years of the life cycle. A trait that hampers
reproductive ability in the young will be quickly eliminated from the
population. A trait that is expressed in the in the older individuals will be
removed much more slowly. As lactose tolerance was beneficial for individuals
within a certain environmental context it got selected irrespective of the
long-term effect it caused.

The
following table may help to explain which traits get naturally selected. Traits
and adaptations may have in all four aspects: a) short term benefit, b) short
term cost, c) long term benefit, and d) long term cost

Traits that
have short (during reproductive years) and long term benefits (longevity) will
get naturally selected so will traits that have short term benefit and a long
term cost. Traits with short term cost will get weeded out if they negatively affect
an organisms’ fit with the environment. Lactose tolerance, in todays changed environmental contexts falls
into the category of traits that have short term benefit and long term cost.

Summary

Thus, the
question, “if lactose tolerance is an adaptation shouldn’t we continue to drink
milk?” has two sub-questions. First, is lactose tolerance an adaptation in
todays environmental context? And second aspect deals with nature of life
history evolution and natural selection. The answer to the first question tell
us that lactose tolerance is does not remain an adaptation in a changed environmental context, moreover
it has become a mal-adaptation due to enabling of chronic consumption of milk.
Dairy consumption has been correlated locally as well as globally with many diseases
of civilization. The answer to the second question addresses the issue that a
trait that confers fitness benefits in reproductive life will tend to get
selected even though it has long term costs.

Summary
Statement: Lactose tolerance is a retrospective adaptation that has the
potential to become mal-adaptive in a changed environmental context.

1 comment:

I think you would do well to read David Buller's "Adapting Minds." Actually, I think everyone doing cultural evolutionary research should read this book, but that's another matter. Because it's a grind to read and because you may find much of it irrelevant to your work, you might just want to read his section of the difference between "adaptive" and "adaptation." His presentation draws a clear distinction between these two concepts that would greatly simplify and clarify your discussion.

To summarize Buller's argument (and I'd be surprised if it were original to Buller), to say that something is an adaptation is to make a claim about how that trait came about. In particular, a trait is an adaptation if, and only if, it came about because it conferred on ancestral individuals an increased ability to survive and reproduce relative to individuals who did not possess that trait. In other words, adaptations are those traits that exist in a contemporary population by virtue of having been selected in previous generations. To say that something is adaptive, by contrast, is to make a claim about what function the trait is serving now. Specifically, something is adaptive if and only if it is incurring relative fitness benefits on individuals who hold the trait.

To put this in terms of your example, lactose tolerance is an adaptation. It arose through natural selection, conferring fitness benefits on individuals living in herding cultures. The question you are trying to address is whether lactose tolerance is currently adaptive. The answer seems to be that it depends. In some environmental contexts it may still be adaptive while in others it might not. The trait remains, however, an adaptation no matter what the current context because of the way it originally came about.

Note that using the terms in this way, not all adaptations are adaptive. Because of changing circumstances, an trait that arose as an adaptation can be neutral in the current environment or even maladaptive. Similarly, not all adaptive traits are adaptations. New traits can arise in a population, for instance, through drift or as spandrels. In such cases, the traits are not adaptations yet they may confer relative fitness benefits on individuals living in the current environment who carry them. If so, then the traits will become adaptations for the descendants who inherit them. Whether they remain adaptive in the descendant population will then depend on environmental circumstances.